Writing fluently readable assertions with AssertJ

The last section of this chapter will explain the fundamentals of AssertJ and explain how to improve verification readability with custom extensions.

Employing assertion chains

In Chapter 4, Testing Exceptional Flow, one of the examples uses three assertXXX statements to check whether:

• An expected exception is not null

• It's an instance of IllegalArgumentException

• It provides a specific error message

The passage looks similar to the following snippet:

Throwable actual = ...

assertNotNull( actual );
assertTrue( actual instanceof IllegalArgumentException );
assertEquals( EXPECTED_ERROR_MESSAGE, actual.getMessage() );

Indeed, it takes a second or two to grasp the verification conditions. This is because there is a lot of redundant clutter: the relevant attributes of the Throwable type are checked one by one, always repeating the "assert" prefix and dispatching the actual parameter to the assertion statements. AssertJ, [ASSERJ], strives to improve...